Glossary terms starting with "D"

A stria in the shape of an elongated chamber. The external wall of the alveolus is perforated by many areolae and the internal wall is perforated by one long opening. Plural form is alveoli. An example genus is Pinnularia.

A German term for “surface growth” or “overgrowth”. A community of small animals and plants that adhere to surfaces in aquatic environments. Green algae and diatoms make up the dominant component of an aufwuchs. Small crustaceans, rotifers, and protozoans are also commonly found in both fresh water and marine aufwuchs faunas, while insect larvae, oligochaetes and tardigrades are peculiar to freshwater aufwuchs faunas. An English pronunciation might be “OWF-vooks”.

A special cell that develops and expands before producing a new frustule. Maximum size of a population is restored through auxospore formation. Auxospores are usually associated with sexual reproduction.

A narrow ridge of silica along the axial area on the inside of the valve bordering the raphe.

Mastogloia smithii is an example which has two axial costae. Together they form the sides of a groove, or gutter (Paddock and Kemp, 1990), widening slightly at the central nodule and stopping short of the distal raphe ends.

A valve end, or apex, having the shape of a head, or the shape of a rounded knob.

Other terms for valve ends include subcapitate (less strongly capitate),
rostrate,
and subrostrate (less strongly rostrate). Subcapitate and subrostrate are quite similar and many taxa may be described as having subcapitate to subrostrate valve ends.

In general, capitate and subcapitate ends are more expanded than the most narrow point on the valve, while rostrate and subrostrate ends are not more expanded than the most narrow point.

In raphid diatoms, the thickly silicified area located between the proximal raphe slits. Often thickened, both in valve and girdle views. It is the site where the silica deposition vesicle deposits the first silica during valve formation.

The series of linking, siliceous bands associated with a
valve.
Each band is called a girdle band, or copula (from Latin for band or link; plural is copulae). Plural form is cingula.

Each valve has its own cingulum and the valves of most diatoms have a cingulum. The epivalve has the epicingulum and the hypovalve has the hypocingulum. The epicingulum always overlaps the hypocingulum. The two cingula together compose the girdle, or cincture.

Girdle bands may be secreted after the vegetative cell division that produced the valve. In other words, while the deposition of the valve is tied to cell division, the deposition of its girdle bands may not be.

A thin flap of silica on the external valve face, lying along the apical axis. Hyaline or finely porous. Proximal margin formed by the edge of the raphe slit. Extends unsupported over the valve face. Distal margin is free.
May lie flat or may be slightly to distinctly elevated. May partially or completely cover the striae. Found in genera such as Fallacia, Lyrella, Mastogloia, Microcostatus, Nitzschia and Sellaphora. Plural form is conopea. From Greek for canopy.

Details of the conopea are best viewed with the scanning electron microscope, but may be visible with the light microscope as longitudinal lines near the raphe or as unornamented areas of the valve face.

Conopea have been found to contain symbiotic nitrogen-fixing cyanobacteria.

Striae that lean toward the central nodule, with the sternum as the origin. Convergent striae may occur throughout the valve, at the valve center, or at the valve apices. In contrast, striae are radiate, or divergent, if they lean away from the central nodule, with the sternum as the origin.

An unornamented, elongated thickening of the valve. Usually refers to a thickening parallel to the striae. Plural form is costae. From Latin for rib.

In some genera, such as Diatoma, Meriodion, and Tetracyclus, costae are further qualified by the extent to which they transverse the valve face. Primary costae fully transverse the valve, whereas secondary costae end before reaching the other side of the valve, and tertiary costae end before reaching the sternum.

An internal valve occurring in species in the genus Craticula. This craticular valve consists of a raphe-sternum and robust transverse bars. Frustules with normal morphology produce the craticular valve under conditions of high solute concentration. The formation of these internal valves is not linked to mitosis.

A thin, domed, porous plate of silica covering the internal opening of an areola, found in genera such as Thalassiosira and Stephanodiscus. A cribrum is a type of
velum.
Visible only with the SEM. Plural form is cribra.

A similar structure with finer pores, found in many raphid diatoms, is the
hymen.

The larger theca of the
frustule. The epitheca overlaps the
hypotheca.
A theca is composed of a
valve
and its associated
cingulum.
From Greek, theca means sheath or case (plural is thecae), so epitheca means outer sheath.

A
hyaline
area of thickened silica extending from the central area of some pennate diatoms. A transverse fascia is formed by secondary deposition of silica into depressions on the valve. Examples of genera having a transverse fascia include Luticola and Staurophora. Plural form is facsiae.

Internal strut that provides structural support to the canal that contains the raphe. Plural form is fibulae. The fibulae extend transapically from the raphe canal to the valve face. Examples of genera having fibulae include Denticula, Nitzschia, and Surirella. From Latin for clasp, pin, or brooch.

The siliceous parts of the diatom cell wall. Composed of the larger
epitheca
and the smaller
hypotheca.
The epitheca overlaps the hypotheca similar to a pill box or Petri dish. From Latin for a little piece.

A tubular process of some centric diatoms. The fultoportula consists of a central tubular process surrounded by two or more satellite pores. Externally, the fultoportula appears as either a tube or a simple pore in the valve wall. The plural form is fultoportulae.

Usually associated with the secretion of §-chitin to maintain buoyancy in the plankton.

The internal, distal termination of the raphe in the shape of a pair of lips or a rolled tongue. The helictoglossa occurs in many raphid diatoms. If present, it can be distinguished in the light microscope in valve view by optical dissection (focusing through many narrow, optical planes). It may also be visible in girdle view as a thickened lip of silica. Plural form is helictoglossae. From Greek for rolled tongue.

Frustules
in which one valve differs from the other morphologically. Frustules may be heterovalvar in their valve ornamentation or their
raphe
system. The term is often applied to the
monoraphid
diatoms, in which one valve has a normal raphe valve and the other has a rapheless valve.

The illustration, collection, or specimen that represents a taxon. 2 - A type specimen. 3 - A type material recognized by the International Code of Nomenclature for algae, fungi, and plants (ICN).

For diatoms, many authors designate an individual frustule or valve on a slide as the type specimen. They do this by imaging this specimen and circling it with a diamond objective marker, or marking its coordinates with a device such as an England Finder. While the ICN allows the whole slide containing the type specimen to be designated as the holotype, diatomists tend to avoid this practice in order to minimize later misinterpretation of the intent of the author.

The ICN uses a system called typification that makes objective identification possible by linking the name of a taxon with a holotype. Typification is intended to create a stable system of nomenclature by tying the published name to a holotype or a suite of holotypes.

Several other kinds of type material are recognized by the ICN, including
isotype,
lectotype, neotype and paratype. Types are not expected to be “typical” or “idealized” specimens, although workers may find such use of the terms.

A silica extension with the shape of a hood on the inside of a valve. It occurs only in some species of Planothidium where it partly covers the inside of the asymmetrical central area of the rapheless valve. See also
rimmed depression.

A very delicate, porous membrane of silica covering the opening of an areola or an alveolus. The pores of the hymen may be round or elongated and range from 5-10 nm (0.005-0.010 µm) in their shortest diameter. Found in many raphid genera such as Cocconeis and Neidiopsis. Visible only with the SEM. Plural form is hymenes.

A similar structure with larger pores, found in many centric and some raphid diatoms, is the
cribrum.

A thickened area of silica without pores on the valve face parallel to the
sternum
and interrupting the striae. One on each side of the sternum connected at the central node, giving the whole structure a lyrate shape (shape of a lyre). A freshwater example occurs in Mastogloia pumila.

A silica projection on a split ring girdle band. The ligula of a younger girdle band fills, or nearly fills, the gap caused by the split in the older band next to it. Found in many diatom taxa. From Latin for strap. Plural form is ligulae.

The acronym for light microscope. The study of diatoms relies on a compound light microscope in which a beam of light passes through optical lenses to view an image of the specimen. The light microscope provides images of the transparent features of the diatom.

Contributors to the Diatoms of the United States project use research grade light microscopes with the minimum specifications of a 100x, 1.3 numerical aperature (NA) oil immersion objective lens and 1.3 NA condenser lens.

A chamber having the shape of a tube inside the valve, oriented along the apical axis. It occurs in genera such as Neidium and Muelleria, which have valves consisting of two layers of silica with the canal lying between them. The canals appear as longitudinal lines and may be central or marginal.

Pennate diatoms having a
raphe
on only one
valve.
A monoraphid diatom is a type of
heterovalvar
diatom. Example genera include Cocconeis, Achnanthidium, and Eucocconeis.

During the ontogeny of monorahid diatoms, a raphe begins to form on each valve. One valve completes its raphe, and becomes the raphe valve. The other valve fills its incipient raphe with silica, leaving a thickened axial rib, and becomes the
rapheless valve.

The genus Nupela includes some monoraphid species and some
biraphid
species. In this web flora, Nupela is included in the biraphid group.

A polysaccharide built of amino sugars and often associated with proteins. Secreted from a variety of openings in the diatom frustule. Mucopolysaccharides can be components of mucilage, a complex colloidal material used by diatoms for movement, protection, and adhesion.

A bulbous chamber on the inside of the
valvocopula.
Found only in the genus Mastogloia. Usually arranged in a row on each side of the valvocopula, together forming the
partectal ring. Plural form is partecta.

The partecta secrete mucilage to the outside of the diatom cell through relatively large pores, the partectal pores, in the wall of the valvocopula. These pores are connected to the partecta by the partectal ducts.

The group of bilaterally symmetric diatoms. This is a heterogeneous group that includes araphid, monoraphid, and biraphid taxa. In older taxonomic systems, pennate diatoms are contrasted with the centric diatoms. Note, however, that these distinctions are artificial, and pennate and centric groups are not natural evolutionary lineages.

The axis of the valve which is perpendicular to the center of the valve face. In centric diatoms, the center is the meeting point of the radii. In pennate diatoms, the center is the meeting point of the apical and transapical axes.